The present invention relates to an Ni base forged alloy and a gas turbine utilizing the same and more particularly to the Ni base forged alloy which is excellent in the segregation property and easy in hot forging and miniaturization of crystal grains and can manufacture large-sized members and the gas turbine utilizing the same.
With high efficiency of a gas turbine, the Ni base forged alloy having the excellent high-temperature strength is used in various members. The Ni base forged alloy much contains solid solution strengthening elements such as W, Mo and Co and precipitation strengthening elements such as Al, Ti and Nb and those elements contribute to the strength of the alloy greatly. Particularly, a γ′-phase (gamma prime phase) made of Ni3Al and a γ″-phase (gamma double prime phase) made of Ni3Nb which are the precipitation strengthening phases can be precipitated to matrix phase finely and innumerably and are extremely effective in improvement of the high-temperature strength. The γ′-phase and γ″-phase are stabilized by Al, Ti and Nb and the design of the high-temperature strength in development of Ni base alloy pays the primary point to the phase stability of the precipitation strengthening phases.
However, the sold solution strengthening elements and the precipitation strengthening elements are apt to be segregated during solidification as these elements are added and it is difficult to manufacture the large-sized members. Accordingly, the use of the high-strength Ni base alloy is mainly limited to members for airplanes and small-sized components such as movable and stationary blades for land. For example, Alloy 718 is widely put to practical use as the Ni base forged alloy having more excellent high-temperature strength by the γ′-phase and γ″-phase, although the segregation property is reduced due to Nb and Mo added thereto and accordingly when it is applied to relatively large-sized members, a manufacturing method of controlling the solidification speed or the like is required. Further, in the manufacturing of large-sized material exceeding 5 tons, the solidification condition has restriction in order to continue stable operation and there are a lot of Ni base alloys to which it cannot be applied.
In JP-A-2012-117122, the segregation property of the Alloy 718 is improved. It is considered that the cause bringing about the segregation is that solute elements are distributed in solid-liquid interface unevenly, so that difference in density in molten metal is changed. The elements having the atomic weight larger and heavier than the molten metal has a tendency that the difference in density in molten metal is smaller as the addition amount is reduced and the segregation is suppressed. Conversely, the element having the light atomic weight has smaller difference in density in molten metal as the addition amount is increased and accordingly there is a tendency that the segregation is suppressed. Accordingly, elements (Al, Ti, Nb, Mo) having the segregation tendency different from one another are balanced to adjust so that the density difference in the molten metal approaches 0 and suppress the segregation, so that the excellent high-temperature strength and the manufacturing of large-sized steel lump can be compatible.
As the others, in order to improve the segregation property, there is also a method of controlling the partition coefficients of elements as described in JP-A-2009-191301. In the element having large density difference between the molten metal and the element, generation of macro segregation is promoted as the partition coefficient is separated from 1, although it has been found that the partition coefficient of particular element can be controlled by changing the addition amount of another element. In JP-A-2009-191301, it has been successfully achieved that Co is added to thereby approach the partition coefficient of W that promotes generation of macro segregation greatly to 1 as well as Al, Ti and Nb of precipitation strengthening elements.
In the above-described two patent documents, the manufacturing upon casting is improved by adjustment of components of alloy. However, in case of forged members, it is necessary to consider not only castability but also forgeability in order to manufacture large-sized steel lump. Generally, forged member is manufactured via the forging process after casting, although molding is difficult as material is larger. The molding uses a method of making forging and rolling in the heated state at high temperature, but the load required for molding is extremely increased in the large-sized high-strength member. Particularly, when the phase in which the strength is increased at high temperature as in γ′-phase in the forging temperature is left in the alloy containing a lot of precipitation strengthening elements, it is impossible to make molding due to excessive distortion resistance or forging crack is sometimes caused even if the load is sufficient. Accordingly, it is necessary to make molding at higher temperature in order to reduce the distortion resistance of material, although when the heating temperature rises, a possibility that material is partially molten and crack is formed is increased. Furthermore, it is necessary to heat material frequently in order to hold the processing temperature and processing time and heating energy are both consumed much.
Further, the forging has the effects that crystal grains are miniaturized and the fatigue strength is improved, although conversely material is held at high temperature, so that grain growth is promoted and crystal grains are coarsened. Accordingly, there is also an aspect that it is difficult to miniaturize the crystal grains in the molding at high temperature for a long time as described above and the reliability of material cannot be ensured.